On being a parasite in an invertebrate host: a short survival course.

Many parasites develop in invertebrate hosts that possess internal defense systems (IDS) that vigorously defined self-integrity. Invertebrates apparently do not produce a large diversity of finely tuned immunorecognition molecules but rather rely on recognition of patterns. As a consequence, requirements for immune evasion are likely to be fundamentally different in such hosts. Although parasites of invertebrates certainly employ diverse tactics to evade host IDS, this review focuses on parasite-mediated interference with the structural and functional integrity of host hemocytes and argues that this is a common strategy of immune evasion. Parasites mediating such effects on host hemocytes are termed suppressors. In some cases, interference is mediated by mutualistic symbionts carried by the suppressors. Hemocytes from infected hosts exhibit diminished adherence to substrates, impaired spreading ability, and reduced ability to participate in phagocytosis or encapsulation reactions. As a result of the action of suppressors, the host's vulnerability to opportunistic parasites is increased, a phenomenon termed acquired susceptibility. A strategy of interference is therefore risky, particularly for suppressors with relatively long development times. As a result, suppressors may provoke either a partial generalized interference or a selective interference with host IDS function, actively contribute to protection of the host to discourage growth of opportunists (termed parasite-mediated internal defense), or induce compensatory host responses that protect the host but that do not jeopardize their own development. Some parasites consistently colonize previously infected hosts and seem to be specialized opportunists.